A well-known method of semiconductor chip fabrication involves depositing epitaxially-grown silicon germanium (SiGe) on a silicon substrate in a chemical vapor deposition (CVD) reactor. This deposition of SiGe on the silicon substrate provides a layer of material to form transistors. SiGe deposition is commonly used to produce high-speed, low-power RF and photonic devices.
During SiGe deposition, oxygen present in the CVD reactor chamber is incorporated into the SiGe film. Although the mechanism for enhanced oxygen incorporation in SiGe films is not fully understood, it is a well-documented phenomenon. Elevated oxygen levels in the CVD reactor chamber used to deposit SiGe cause numerous problems in the SiGe films produced in the CVD reactor chamber. Among these problems are elevated sheet resistance of the SiGe p-type base and poor crystal quality. In addition, the reactor must be taken offline for weeks or even months before acceptable levels of oxygen in the chamber are achieved; while the reactor is offline, it is completely disassembled to remove moisture, reassembled, and then tested until acceptable oxygen levels in SiGe films are achieved. This is costly as the offline reactor clearly cannot be used for manufacturing semiconductor chips.
Reducing and stabilizing oxygen levels in the CVD reactor is desirable because it would reduce variation in resistance of the SiGe p-type base, improve crystal quality, and also increase the amount of time a single CVD reactor is online and used for manufacturing chips. In addition, minority carrier lifetime increases by 1.33 orders of magnitude for each decade of oxygen reduction. (T. Ghani et al., “Effect of Oxygen on Minority-Carrier Lifetime and Recombination Currents in Si1-xGex Heterostructure Devices,” Applied Physics Letters, 58(12), 1991) Increasing the minority carrier lifetime would improve the performance of the semiconductor chip by increasing the number of charge carriers.
Efforts to reduce the effects of elevated oxygen levels or SiGe base resistance in CVD reactors have focused on adding diborane (B2H6) to the reaction chamber. While the addition of boron does reduce the sheet resistance of the SiGe p-type base, it may require other process adjustments which would ultimately have a negative effect on the process's stability. For instance, it has been demonstrated that variations to B2H6 flow can modulate the SiGe base width and Ge concentration.
It is known that the elevated oxygen levels in a CVD reactor are due to outgassing and permeation of gas (moisture and solvents) from the sealing o-rings used on a CVD reactor. Outgassing is a result of gas, usually water vapor, desorbing from the CVD reactor chamber-side surface of the o-ring as well as gas in the bulk of the o-ring diffusing to the surface of the o-ring, where it is desorbed. Gas from the atmosphere can permeate across the bulk of the o-ring via diffusion transport and into the reactor chamber by desorbtion. (P. Danielson, “Gas Loads and O-Rings,” A Journal of Practical and Useful Vacuum Technology) Once moisture is in the reactor chamber, it will adsorb and desorb over and over.
Permeation is a function of material properties of the o-rings, how many linear inches of the o-rings are exposed to vacuum, and the pressure differential across the membrane or seal. While the gas load resulting from permeation is constant, the gas load resulting from outgassing and virtual leaks fluctuates; generally, permeation rates determine the lower boundary of oxygen concentration in the CVD reactor while outgassing rates set the upper boundary of oxygen concentration in the reactor. O-ring permeation can be described mathematically by the following equation:
      Q    =          K      ⁢                                    P            1                          1              /              j                                -                      P            2                          1              /              j                                      h              ,where h is the effective material thickness, K is the permeation constant, and j is the dissociation constant (generally, j=1 for gases in non-metals, j=2 for diatomic gas in metal).
As shown in FIG. 1 (taken from Phil Danielson, “Gas Loads and O-Rings,” A Journal of Practical and Useful Vacuum Technology, 2002), when a new unbaked o-ring is installed on a vacuum system, the outgassing via diffusion transport of the unbaked o-ring 10 is responsible for a gas load higher that that due to permeation 12. As pumping time in the vacuum system increases, the gas load due to outgassing from the unbaked o-ring 10 decreases and permeation 12 becomes responsible for the primary gas load. The gas load due to permeation 12 remains constant. The gas load due to outgassing from a baked o-ring 14 over time becomes lower than that due to permeation 12 but, as will be noted in greater detail below, baked o-rings may be unsuitable as sealing rings in CVD reactors due to effects to elasticity and potentially the permeation properties.
In SiGe deposition, outgassing from the o-rings may affect the stability of the process and therefore frequently requires adjustments to the process. It would be desirable to reduce the oxygen concentration in the CVD reactor to levels due to permeation since, as noted above, oxygen levels due to permeation are constant over time and generally are lower than oxygen levels due to outgassing.
The gas load from the o-rings is due in large part to the manufacturing process. A new o-ring may contain unreacted monomer, solvents, volatile curing agents, and water vapor. The curing process may also increase the gas load since HF is formed during curing and acid acceptors such as MgO are added to react with HF. (Id.) It can take weeks or months for the water in a new ring to outgas; however, in an oxygen-sensitive process such as SiGe deposition in a CVD reactor, this can be extremely expensive since it removes the CVD reactor from the manufacturing process until acceptable oxygen levels are achieved. O-rings may be baked at atmosphere before installation but this only removes some of the water trapped in the o-ring bulk and may affect the elasticity of the o-ring, making it unsuitable for sealing the CVD reactor. The vacuum baking will remove a large portion of moisture but the effect on elasticity, mass loss, and permeation rates are a serious concern.
Although vacuum and/or heat-treated o-rings may be purchased from vendors, the gas load in these o-rings is still high. The lowest recorded level of oxygen in SiGe film using low-moisture o-rings per the vendor specifications for preparation (wiping the o-ring with isopropyl alcohol before installation) is 1018 atoms/cc of oxygen; this oxygen level was achieved only after approximately 1500 wafers had been processed over the course of more than five weeks. Given the amount of time it takes for a new o-ring to outgas, the SiGe deposition process may be unstable for weeks following a preventive maintenance procedure where a new o-ring is installed before the oxygen concentration in a CVD reactor to reach an acceptable level and consistent performance is achieved. Therefore, it would be desirable to have moisture-depleted o-rings that could be installed on a CVD reactor without requiring the reactor to be offline for a long period of time until acceptable oxygen levels are reached.
It is an object of this invention to provide a preparation for sealing o-rings that removes excess moisture from the bulk of the o-ring without affecting the o-ring's elasticity, mass, and permeation rate.
It is an object of this invention to provide o-rings with a low gas load.
It is another object of the invention to provide o-rings for CVD reactors which will not create elevated oxygen levels in the reactor chamber due to outgassing.
It is yet another object of this invention to provide o-rings for use with CVD reactors so that oxygen levels in a CVD reactor are close to permeation levels which remain constant over time.
It is yet another object of this invention to provide moisture-depleted o-rings for use with a CVD reactor such that the oxygen concentration in SiGe films are reduced.